In the past, an entire lease was usually filed of record, but in more modern times, you will often see a memorandum (memo) filed in place of the lease itself. A memo is basically a summary of a lease.
A memo provides notice of a lease without disclosing confidential information. It is a concept somewhat similar to that of a deed that references “$1 and other good and valuable consideration”. The grantee in such a deed did not only pay $1 for the property. That phrase is used in the deed to show that consideration was paid (without which the deed would not be valid), but keeps private the actual amount. Likewise, a memo lets others know that a lease was taken but keeps private information relating to payments and/or royalties.
When negotiating a lease with a landowner, be aware that a lease and memo will generally both be included in the lease packet for the landowner to sign. When you come across one in your chain of title, you can treat it the same as if you had come across an oil and gas lease.